Man how the hell do you keep a cat indoors? I'm keeping Guts in, and he's growing to hate me. :(

Man how the hell do you keep a cat indoors? I'm keeping Guts in, and he's growing to hate me. :(
They get used to it eventually; generally speaking I've never let a cat outdoors on purpose, and they don't even treat the front door like a door anymore.
Yeah, my Guts is jet-black, and we're in the middle of the city. I physically cannot let him out, he'll get obliterated by a bike messenger or a shitty driver.
You can get a cat leash harness and take your cat on supervised enrichment walks.
Otherwise? Just make the interior more enriching by adding stuff he likes to it (especially explorables like boxes, tubes, scratchposts)